1. Field of the Invention
The present invention relates to a novel process/catalyst for the direct synthesis of dimethyldichlorosilane.
2. Description of the Prior Art
The industrial process for the manufacture of organochlorosilanes and in particular of dimethyldichlorosilane, hereinafter designated DMCS, is well known to this art and is particularly described in U.S. Pat. No. 2,380,995, as well as in the text by Walter Noll, Chemistry and Technology of Silicones, pages 26-41, published by Academic Press Inc. (1968).
According to this so-called "direct synthesis" or "Rochow synthesis", organochlorosilanes, and in particular DMCS, are directly produced by reacting methyl chloride with a solid contact mass of silicon and of a catalyst containing copper, according to the reaction: EQU 2CH.sub.3 Cl+Si.fwdarw.(CH.sub.3).sub.2 SiCl.sub.2
In actual fact, other products, especially methyltrichlorosilane CH.sub.3 SiCl.sub.3, hereinafter designated MTCS, and trimethylchlorosilane (CH.sub.3).sub.3 SiCl, hereinafter designated TMCS, are formed during the direct synthesis.
Other by-products are also formed, such as, for example, (CH.sub.3)HSiCl.sub.2 and (CH.sub.3).sub.2 HSiCl and heavy products which are polysilanes, essentially disilanes.
Among all of such products produced by direct synthesis, DMCS is the most desired compound. After hydrolysis and polymerization, this compound enables the preparation of oils and resins which are basic starting materials for the production of silicones. Thus, DMCS is used for the preparation of polyorganosiloxane resins, as described in U.S. Pat. Nos. 2,258,218 to 2,258,222, for the preparation of oils as described in U.S. Pat. Nos. 2,469,888 and 2,469,830 and for the preparation of polyorganosiloxane elastomers as described in U.S. Pat. No. 2,448,756.
It is also known to this art to employ copper or compounds of copper as a catalyst for the above direct synthesis reaction, employed in the form of an alloy or mechanical admixture with silicon, optionally deposited onto an inorganic support.
To improve the DMCS yield, it too has been proposed to this art to add to the copper a promoter system containing one or more additives. These additives include zinc or a zinc halide (U.S. Pat. No. 2,464,033), aluminum (U.S. Pat. Nos. 2,403,370 and 2,427,605), tin, manganese, nickel and silver (British Pat. No. 1,207,466), cobalt (British Patent No. 907,161), potassium chloride (Russian Patent No. 307, 650), phosphorus or a phosphorus compound (U.S. Pat. No. 4,602,101), or arsenic or an arsenic compound (U.S. Pat. No. 4,762,940).
Such promoter systems based on the aforementioned additives undoubtedly improve the direct synthesis process, but they nevertheless present at least one of the following disadvantages:
(1) The selectivity for DMCS, evaluated as the average weight ratio MTCS/DMCS and/or as the molar % of DMCS relative to the total amount of the silanes produced, remains insufficient;
(2) The induction time and the initiation temperature of the reaction are too high;
(3) The mean activity of the catalyst system, also referred to as production efficiency, evaluated as the weight of methylchlorosilanes (MCS) produced per hour and per kg of silicon introduced, and the maximum degree of conversion o silicon remain insufficient;
(4) The catalyst system is sensitive to impurities; and
(5) The formation of by-products and, in particular, of disilanes remains high.
European patents EP-A-0,138,678 and EP-A-0,138,679 describe the use of a copper catalyst comprising an improved promoter system which contains:
(i) 30 to 1000 ppm (calculated as the weight of metal relative to the weight of silicon employed) of at least one metal selected from tin and antimony or of a compound based on tin and/or antimony,
(ii) 0.05% to 2% (calculated as indicated above) of at least one alkali metal selected from among lithium, sodium, potassium, rubidium and cesium or of a compound based on the same alkali metals, and
(iii) optionally, 0.1% to 3% (calculated as indicated above) of metallic zinc or of a zinc-based compound.
When compared with a promoter system not comprising any alkali metal, the promoter systems described in the aforesaid European patents enable, when the reactions are used in one case and in the other case at appropriate temperatures and for similar periods of time resulting in equivalent consumptions of the Si introduced, attaining at least an appreciable increase in the selectivity for DMCS, evaluated, on the one hand, as the average weight ratio MTCS/DMCS and, on the other, as the molar % of DMCS relative to the total amount of silanes produced.
However, despite the advantages presented by the prior art catalysts and, in particular, those described in the aforesaid European patents, serious need continues to exist in this prior art for even further improved and/or more versatile catalyst systems which, depending on the particular reaction conditions selected to carry out the above direct synthesis on a industrial scale, permit determining with greater certainty the compound which is best suited to a given need.